Gate Delays (Propagation Delays)

All physical circuits exhibit a propagation delay between the transition of an input and a resulting transition of an output.

The propagation delay (or gate delay) is the length of time which starts when the input to a logic gate becomes stable and valid to change, to the time that the output of that logic gate is stable and valid to change.

Because each logic gate in a circuit has a propagation delay, a signal transition at the input of a circuit cannot immediately cause a change in the logic value of the output of a circuit. Propagation delays ultimately limit the speed at which a circuit can operate.

The determination of the propagation delay of a combined circuit requires identifying the longest path of propagation delays from input to output and by adding each propagation delay along this path.

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  • Sequential Logic Circuits

    If the clock (synchronizing) pulses arrive at a regular interval, the combinational logic must respond to a change in the state of the flip-flop in time to be updated before the next pulse arrives. Propagation delays play an important role in determining the minimum interval between the clock pulses that will allow the circuit to operate correctly.

    The storage capability of a time-delay device varies with the time it takes for the signal to propagate through the device. In practice, the internal propagation delay of logic gates is of sufficient duration to produce the needed delay, so that actual delay units may not be necessary.

  • Forms of Boolean Algebra

    In general, a two-level implementation is preferred because it produces the least amount of delay through the gates when the signal propagates from the inputs to the output. However, the number of inputs to a given gate might not be practical.

  • Flip-Flops

    The propagation delay time of the flip-flop is defined as the interval between the trigger edge and the stabilization of the output to a new state.

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